Light emitting device

ABSTRACT

It is an object of the invention to provide a light emitting device in which burden on a light emitting element having low luminous efficiency is relieved, and the deterioration of a light emitting element, the reduction in color reproduction due to the deteriorated light emitting element, and increase in electric power consumption can be suppressed. A light emitting device according to the invention has light emitting elements each of which emits one of colors corresponding to three primary colors. Further, one feature of the light emitting device according to the invention has a light emitting element which emits a neutral color. The light emitting device according to the invention has a structure in which a plurality of pixels having light emitting elements each of which emits one of colors corresponding to three primary colors, and a light emitting element which emits a neutral color as one group, are arranged.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device, in particular,to a light emitting device which can display an image which includesmulticolors by combining light emission having different colors.

2. Description of the Related Art

A light emitting device utilizing light emitted from anelectroluminescence element (a light emitting element) is a device whichhighly draws attention as a display device having wide view angles andlow power consumption.

In recent years, in a field of developing a light emitting device, theresearch and development of a light emitting device capable ofdisplaying a high quality full color image has been accelerated toensure the market of display devices and the like for variousinformation processing devices such as a television receiver or a carnavigation system.

In order to display a full color image, it is required that regionsemitting light emission of three primary colors of red (R), green (G)and blue (B) are independently provided so as to control brightness ofcolor and chromaticity to be indicated by changing light emission timeand light emission brightness in each region.

As a method for changing brightness of color and chromaticity, there area method for changing the brightness of color and chromaticity bychanging light emission time of a light emitting element, and a methodfor changing the brightness of color and chromaticity by changingbrightness of a light emitting element. In the case of employing theformer method, brightness of color and chromaticity are changed byvariously combining each light emission of red, green and blue bychanging light emitting time according to each luminescent color.

Meanwhile, luminous efficiency of a light emitting element differsaccording to a luminous body included in the light emitting element andcharacteristics of other substances. In a light emitting device,luminous efficiency differs in each light emitting element whichindicates each luminous color. Accordingly, more current is relativelyrequired in a light emitting element having lower luminous efficiency toobtain light emission having desired brightness.

Furthermore, the human eye has different sensitivity to each emissionwavelength, and generally has higher sensitivity to the emissionwavelength of green than that of red or blue. Hence, it is required tomake brightness of blue and red relatively higher than that of green sothat blue and red emit light to which human eye has the same sensitivityas green.

Flowing a lot of current to a light emitting element to increasebrightness of the light emitting element make the light emitting elementpromotes the deterioration of the light emitting element, and leads toincrease in electric power consumption of a display device. What ismore, when emission wavelength shifts due to a deteriorated lightemitting element, color reproduction of a light emitting devicedecreases and thus sometimes image quality is deteriorated. Therefore,the development of a luminous body or a light emitting element which canemit light efficiently and has longer life has been attempted. Forexample, in Reference 1 (Japanese Patent Laid-Open No. Hei 2002-299062),it is contrived device to enhance luminous efficiency by adjusting lightpath length is carried out.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a light emitting device inwhich burden on a light emitting element having low luminous efficiencyis relieved, and the deterioration of a light emitting element, thedeterioration of color reproduction due to the deteriorated lightemitting element, and increase in electric power consumption can besuppressed.

A light emitting device according to the invention has light emittingelements each of which emits one of colors corresponding to threeprimary colors. Further, one feature of the light emitting deviceaccording to the invention has a light emitting element which emits aneutral color. The light emitting device according to the invention hasa structure in which a plurality of pixels having light emittingelements each of which emits one of colors corresponding to threeprimary colors, and a light emitting element which emits a neutral coloras one group, are arranged.

In each pixel, a plurality of colors having different brightness ofcolor and chromaticity are displayed by changing and combining lightemitting time or brightness of light emission from each light emittingelement included in the pixel. Note that at least one light emittingelement which emits a neutral color may be included at in one pixel.

Three primary colors are three colors of red, green and blue. Hereupon,red means a color having coordinates in the region where x in achromaticity diagram is 0.6 or more, and y is 0.35 or less, when it isindicated by a CIE-XYZ color system. Further, green means a color havingcoordinates in the region where x in a chromaticity diagram is 0.3 orless, and y is 0.6 or more, when it is indicated by a CIE-XYZ colorsystem. Moreover, blue means a color having coordinates in the regionwhere x in a chromaticity diagram is 0.15 less, and y is 0.2 or less,when it is indicated by a CIE-XYZ color system. Note that the CIE-XYZcolor system is a color system based on three stimulation values X, Yand Z. The chromaticity diagram is a diagram which indicates colors withx-y coordinate space based on three stimulation values x, Y and Z. Notethat chromaticity numerically limits color types of light except forbrightness.

A neutral color means a color having coordinates in a region which isdifferent from above-mentioned regions indicating red, green and blue inthe chromaticity diagram, when it is shown with the CIE-XYZ colorsystem.

A light emitting element which emits a neutral color plays an auxiliaryrole with respect to a light emitting element having low luminousefficiency, when, a neutral color is indicated by combining lightemission of a light emitting element having higher luminous efficiencyand light emission of the light emitting element having low luminousefficiency. Accordingly, in a light emitting element having low luminousefficiency, a burden on light emission is relieved since brightnessrequired for display can be reduced compared with the case of displayinga neutral color by combining the three primary colors. Thus, the life ofa light emitting element having low luminous efficiency can beparticularly extended. As a result, the deterioration of a lightemitting element or the reduction image quality caused by thedeteriorated light emitting element can be suppressed. Further, morecolors can be indicated by providing a light emitting element whichemits a neutral color, therefore, the range of color reproduction rangeof a light emitting device is widened.

The invention relieves the burden on a light emitting element having lowluminous efficiency or a light emitting element of a luminescent colorto which the human eye has lower sensitivity. Accordingly, deteriorationof the light emitting element or the reduction in image quality due tothe deteriorated light emitting element can be suppressed. Moreover, thelight emitting device which can indicate more colors resulting inexpanding color reproduction range can be obtained.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram illustrating a pixel provided for a light emittingdevice according to a certain aspect of the present invention, and thearrangement of light emitting elements which constitute the pixel;

FIG. 2 is a diagram illustrating light emitting elements provided for alight emitting device according to a certain aspect of the invention andcircuits for driving the light emitting elements;

FIG. 3 is a frame format which shows a top view of a light emittingdevice to which a certain aspect of the invention is applied;

FIG. 4 is a diagram illustrating light emitting elements provided for alight emitting device to which a certain aspect of the invention isapplied and circuits for driving the light emitting elements;

FIG. 5 is a top view of a pixel portion of a light emitting device towhich a certain aspect of the invention is applied;

FIG. 6 is a diagram illustrating the frame movement with time course ofa light emitting device to which a certain aspect of the invention isapplied;

FIG. 7 is a diagram explaining pixels provided for a light emittingdevice and the arrangement of light emitting elements constituting thepixels to which a certain of the invention is applied;

FIGS. 8A to 8C are views illustrating cross-sectional structures of alight emitting device to which a certain of the invention is applied;

FIGS. 9A and 9B are views illustrating cross-sectional structures of alight emitting device to which a certain of the invention is applied;

FIG. 10 is a diagram illustrating a light emitting element provided fora light emitting device and circuits for driving the light emittingelement to which a certain of the invention is applied;

FIG. 11 is a top view of a pixel portion of a light emitting device towhich a certain of the invention is applied;

FIG. 12 is a cross-sectional view after sealing a light emitting deviceto which a certain of the invention is applied;

FIG. 13 is a view showing an electronic device to which a light emittingdevice manufactured by applying a certain of the invention is mounted;and

FIG. 14 is a figure illustrating colors with x-y coordinates space basedon three stimulation values X, Y and Z.

DETAILED DESCRIPTION OF THE INVENTION EMBODIMENT MODE

One aspect of a light emitting device according to the present inventionis described with reference to FIGS. 1, 2 and 14. FIG. 14 is achromaticity diagram which indicates colors with x-y coordinate spacebased on three stimulation values X, Y and Z.

A light emitting device according to the invention includes a firstlight emitting element 101 which emits red light, a second lightemitting element 102 which emits green light, a third light emittingelement 103 which emits blue light, and a fourth light emitting element104 which emits blue-tinged green light. Note that a luminescent colorof the fourth light emitting element is not limited to theabove-mentioned luminescent colors and it may be, for example,red-purple, yellow-orange or the like.

Hereupon, red means a color having coordinates in a region where x in achromaticity diagram is 0.6 or more and y is 0.35 or less (a regionsurrounded by the periphery of the chromaticity diagram, and dottedlines 151 and 152 in FIG. 14), when red is indicated by a CIE-XYZ colorsystem. Green means a color having coordinates in a region where x in achromaticity diagram is 0.3 or less and y is 0.6 or more (a regionsurrounded by the periphery of the chromaticity diagram, and dottedlines 155 and 156 in FIG. 14), when green is indicated by a CIE-XYZcolor system. Blue means a color having coordinates in a region where xin a chromaticity diagram is 0.15 or less and y is 0.2 or less (a regionsurrounded by the periphery of the chromaticity diagram, and dottedlines 153 and 154 in FIG. 14), when blue is indicated by a CIE-XYZ colorsystem. Additionally, a neutral color between green and blue means acolor having coordinates in a region where x in a chromaticity diagramis 0.1 or less and y is 0.25 or more and 0.5 or less (a regionsurrounded by the periphery of the chromaticity diagram, and dottedlines 161, 162 and 163 in FIG. 14), when the neutral color between greenand blue is indicated by a CIE-XYZ color system. Preferably, it is acolor which has coordinates in a region where x in a chromaticitydiagram is 0.1 or less and y is from 0.35 or more and to 0.45 or less.

As shown in FIG. 1, a plurality of pixels 110 in which the first lightemitting element 101, the second light emitting element 102, the thirdlight emitting element 103 and the fourth light emitting element 104 arein one group, are arranged.

In FIG. 1, each of the first light emitting element 101 to the fourthlight emitting element 104 is arranged in rows. However, the way ofarranging the light emitting elements is not particularly limited. Forexample, each of the light emitting elements may be arranged in columns,or may be arranged so that the light emitting element which emits redlight is adjacent to the light emitting element which emits blue light.Further, the shape of each light emitting elements is not limited to arectangle as shown in FIG. 1, and for example, it may be a square,another polygon or a shape having curvature.

As shown in FIG. 2, circuits to drive each of the first light emittingelement 101 to the fourth light emitting element 104 is connected thefirst light emitting element 101 to the fourth light emitting element104.

The circuits connected to each light emitting element includes a drivingtransistor 121 that determines a light-emitting or non-light-emittingstate of the light emitting element in accordance with an image signal,a switching transistor 122 that controls an input of the image signal,an erasing transistor 123 that controls the light emitting element to bea non-light-emitting state regardless of the image signal, a sourcesignal line 131, a current supply line 132, a first scanning line 133and a second scanning line 134.

Here, a driving method when the second light emitting element 102 emitslight is described. When the first scanning line 133 is selected in awriting period, the switching transistor 122 that has a gate connectedto the first scanning line 133 is turned on. Then, when an image signalinputted to the source signal line 131 is inputted to a gate of thedriving transistor 121 through the switching transistor 122, currentflows from the current supply line 132 to the second light emittingelement 102 to emit green light. At this time, light emission brightnessis determined in accordance with the current value flowing to the secondlight emitting element 102.

The first light emitting element 101, the third light emitting element103 and the fourth light emitting element 104 are also driven in thesame manner as the second light emitting element 102. Thus, lightemitting time of each light emitting element is separately controlled bythe circuits connected to each light emitting element; therefore, adesired display color can be obtained. Hereupon, the display color meansa color in which light emission obtained from a plurality of lightemitting elements, which are included in a pixel and each of which has adifferent luminescent color, is combined and visually mixed.

Note that a driving method is not limited to the method shown in thisembodiment mode, and a digital driving method other than theabove-mentioned driving method may be employed. Additionally, thecircuits may be operated by an analog driving method.

There is also no limit to an element structure of each transistor, andeither a stagger type or an inverse stagger type may be used. Inaddition, either a single gate structure or a multigate structure may beused. Further, an LDD (Lightly Doped Drain) structure, a single drainstructure or the like may be used.

In the light emitting device described above, blue-tinged green, namely,the fourth light emitting element 104 which emits a neutral colorbetween blue and green plays an auxiliary role with respect to a lightemitting element having low luminous efficiency, when, for example,luminous efficiency of the second light emitting element 102 and thethird light emitting element 103 are different in the case of displayinga neutral color by combining light emission from each light emittingelement. Accordingly, a burden on an element having low luminousefficiency is relieved, and thus the life of the elements can beextended compared with the case of displaying a neutral color bycombining only three primary colors. Therefore, a light emitting devicein which defective indication due to a deteriorated element issuppressed can be obtained. Further, more colors can be indicated byproviding the fourth light emitting element 104; therefore, the range ofcolor reproduction range of a light emitting device is expanded.

Additionally, when the fourth light emitting element 104 emitsblue-tinged green as in this embodiment mode, another advantageouseffects also include the following. It is necessary to make blue lighthave higher brightness compared with green light, for example, in orderto make the human eye have equal sensitivity to green light and bluelight, since the human eye usually has higher sensitivity to green lightIn other words, in a display behavior, the third light emitting element103 which emits blue light has comparatively a greater burden comparedwith the second light emitting element 102 which emits green light.However, as the light emitting device according to the invention, aburden on the third light emitting element 103 can be relieved, sincethe fourth light emitting element 104 plays an auxiliary role withrespect to the third light emitting element 103 by providing the fourthlight emitting element 104 which emits blue-tinged green.

As described above, deterioration of a light emitting element or thereduction in image quality due to the deteriorated light emittingelement can be suppressed by relieving a burden on a light emittingelement with low luminous efficiency or a light emitting element of aluminescent color to which the human eye has lower sensitivity.

Embodiment 1

In this embodiment, a fight emitting device according to the presentinvention is describe However, the light emitting device according tothe invention is not limited to the one shown in this embodiment.

FIG. 3 is a frame format showing a top view of a fight emitting deviceto which the invention is applied. In FIG. 3, reference numeral 6510shown by a dotted line denotes a drive circuit portion (a source sidedrive circuit); 6511, a pixel portion; and 6512, a drive circuit portion(a gate side drive circuit). The light emitting element of the inventionis provided for the pixel portion 6511. The driver circuit portions 6510and 6512 are connected to each other through an FPC 6503 which is anexternal input terminal and one group of wirings formed over a substrate6500. Signals are inputted into the drive circuit portions 6510 and 6512by receiving a video signal, a clock signal, a start signal, a resetsignal or the like from the FPC (flexible printed circuit) 6503. Aprinted wiring board (PWB) 6513 is mounted to the FPC 6503. The drivecircuit portion 6510 is provided with a shift resistor 6515, a switch6516, memories (latches) 6517 and 6518, and the driver circuit portion6512 is provided with a shift resistor 6519 and a buffer 6520.

The drive circuit portions are not necessarily provided over the samesubstrate as the pixel portion 6511 as described above, and the drivecircuit portion may be provided outside the substrate by using, forexample, an FPC, over which a wiring pattern is formed, mounting an ICchip thereon (TCP) or the like. A circuit structure of the drive circuitportions 6510 and 6512 is not limited to the above structure and astructure in which a circuit having a different function from the aboveis further provided may be adopted.

In the pixel portion 6511, a plurality of source signal lines 331 and aplurality of current supply lines 332 extended in columns are arrangedin rows. A plurality of first scanning lines 333 and a plurality ofsecond scanning lines 334 extended in rows are arranged in columns. Aplurality of pixels 301 in which a first light emitting element 301 a, asecond light emitting element 301 b, a third light emitting element 301c and a fourth light emitting element 301 d are in one group, arearranged in matrix.

Further, in a pixel 310, the first light emitting element 301 a, thesecond light emitting element 301 b, the third light emitting element301 c and the fourth light emitting element 301 d are arranged in rowsin the same manner as the light emitting device shown in FIG. 1.

Each light emitting element has a structure in which a light emittinglayer including a luminous body is sandwiched between a pair ofelectrodes. The light emitting layer may be a single layer constitutedby only a layer including a luminous body, or may be a multi-layerformed of a plurality of layers in which a layer including a luminousbody and a layer containing a substance having superior carrier (anelectron, a hole) transportability, a substance having superior carrierinjectability, or the like, are combined.

The first light emitting element 301 a includes4-dicyanomethylene-2-methyl-6-(1,1,7,7-tetramethyldurolysyl-9-enyl)4H-pyran(abbreviation: DCJT) as a luminous body and emits red light. The secondlight emitting element 301 b includes N,N′irethylquinacridon(abbreviation: DMQd) as a luminous body and emits green light. The thirdlight emitting element 301 c includes 9,9′-bianthryl as a luminous bodyand emits green light The fourth light emitting element 301 d includescoumarin 30 as a luminous body and emits blue-tinged green light. Inaddition, the luminous body included in each fight emitting element isnot limited to the above-mentioned luminous body and another luminousbody may be used. For example, as a luminous body which emits red light,4-dicyanomethylene-2-t-butyl-6-(1,1,7,7-tetramethyldurolysyl-9-enyl)4H-pyran(abbreviation: DPA), Peri-firan-ten,2,5-dicyano-1,4-bis(10-methoxy-1,1,7,7-tetramethyldurolysyl-9-enyl)benzene,or the like may be used. As a luminous body which emits green light,coumarin 6, coumarin 545T, tris(8-quinolinolate)aluminum (abbreviation:Alq) or the like may be used. As a luminous body which emits blue light,9,10-diphenylanthracene (abbreviation: DPA),9,10-bis(2-naphthyl)anthracene (abbreviation: DNA), or the like may beused As a luminous body which emits blue-tinged green light,bis(2-methyl-8-quinolinolate)4-phenylphenolate-aluminum (abbreviation:BAlq), bis(2-methyl-8-quinolinolate)4-phenylphenolate-gallium(abbreviation: BGaq), or the like may be used.

As shown in FIG. 4, circuits are connected to the first light emittingelement 301 a to the fourth light emitting element 301 d for drivingeach light emitting element. Each of the circuits includes drivingtransistors 321 (321 a, 321 b, 321 c and 321 d) that determine alight-emitting or non-light-emitting state of each of the first lightemitting element 301 a to the fourth light emitting element 301 d inaccordance with an image signal, switching transistors 322 (322 a, 322b, 322 c and 322 d) that control an input of the image signal, erasingtransistors 323 (323 a, 323 b, 323 c and 323 d) that control each of thefirst light emitting element 301 a to the fourth light emitting element301 d to be a non-light-emitting state regardless of the image signal.Here, sources (or drains) of the switching transistors 322 are connectedto the source signal fines 331, sources of the driving transistors 321and sources of the erasing transistors 323 are connected to the currentsupply lines 332 (332 a, 332 b, 332 c and 332 d) extending to juxtaposeto the source signal lines 331 (331 a, 331 b, 331 c and 331 d), gates ofthe switching transistors 322 are connected to the first scanning line333, and gates of the erasing transistors 323 extending to juxtapose tothe first scanning line 333 are connected to the second scanning line334. Each of the driving transistors 321 (321 a, 321 b, 321 c and 321 d)is serially connected to each of the first light emitting element 301 ato the fourth light emitting element 301 d. A structure of circuitsconnected to each light emitting element is not limited to the structuredescribed here, and another structure different from the above may beused.

FIG. 5 shows a top view of the pixel portion 6511 of a light emittingdevice of this embodiment mode. In FIG. 5, only a part of the pixelportion 6511 is illustrated. Note that a structure of a pixel portion ofthe light emitting device is not limited to the structure shown in FIG.5, and another structure may be used. In FIG. 5, reference numeral 81denotes a semiconductor layer, 82, a conductive film functioning as agate (a gate electrode) of a driving transistor 321, a switchingtransistor 322, an erasing transistor 323, a first scanning line 333, asecond scanning line 334 and the like; and 83, a conductive filmfunctioning as a source signal line 331, a current supply line 332 andthe like. In addition, reference numeral 84 denotes a part which has alaminated structure in which a light emitting layer is sandwichedbetween a pair of electrodes.

Hereinafter, the operation of the light emitting device of thisembodiment is described with reference to FIG. 6. FIG. 6 is a figure todescribe the frame movement with time course. In FIG. 6, the horizontaldirection indicates time course and the vertical direction indicates ascanning direction of a scanning line.

In the light emitting device according to this embodiment, as shown inFIG. 6, time-sharing is performed on a frame to have four subframes:501, 502, 503 and 504 which include write periods 501 a, 502 a, 503 aand 504 a, and store periods 501 b, 502 b, 503 b and 504 b. A lightemitting element provided with a signal for emitting light is in a lightemitting state in the store period. The ratio of store period length ineach subframe is, the first subframe 501: the second subframe 502: thethird subframe 503: the fourth subframe 504 equal to 2³:2²:2¹:2⁰ equalto 8:4:2:1. Accordingly, 4 bits gradation can be expressed. However, thenumber of bits and the number of gradations is not limited hereto, andfor example, 8 subframes may be provided to perform 8 bit gradation.

Movement in one frame will be described. First, in the subframe 501,write movement is sequentially performed from a first line to the lastline. Hence, the start time of write period differs according to lines.A line in which the write period 501 a is finished sequentially turns tothe store period 501 b. In the store period, a light emitting elementprovided with a signal for emitting light is in a light emitting state.A line in which the store period 501 b is finished sequentially turns tothe next subframe 502, and as in the subframe 501, write movement issequentially performed from a first line to the last line. By repeatingthe above-mentioned movement, to the store period 504 b in the subframe504 is finished. When movement in the subframe 504 is finished, movementis turned to the next frame. In this way, integrated time of lightemission in each subframe is equivalent to light emitting time of eachlight emitting element in one frame. By changing the light emitting timein each light emitting element and variously combining the lightemitting elements in a pixel portion, diverse display colors in whichbrightness and chromaticity differ can be formed.

In a subframe 504 in which a store period is shorter than write periods501 c, 502 c and 503 c in which from a write period of the first line toa write period of the last line are included, an erase period 504 d isprovided after the store period 504 b to compulsorily make the subframe504 in a non-light-emitting state. Accordingly, the write period of thesubframe 504 and the write period of the next subframe thereof can beprevented from being superposed with each other.

In this embodiment, the subframes 501 to 504 are sequentially disposedfrom the one having the longer store period. However, the subframes arenot necessarily disposed as in this embodiment For example, thesubframes may be sequentially disposed from the one having the shorterstore period, or the one having the longer store period and the onehaving the shorter store time may be disposed in random.

Then, circuit operation in a write period is explained. In a writeperiod, the first scanning line 333 in the n-th line (n is naturalnumber) is selected, and the switching transistor 322 connected to thefirst scanning line 333 turns on. At this time, a video signal issimultaneously inputted to the source signal lines of from the firstline to the last line. However, each of the video signals inputted fromthe source signal lines 331 is independent with each other. The videosignals simultaneously inputted from the source signal lines 331 areinputted to the gates of the driving transistors 321 through theswitching transistors 322. At this time, according to the signalsinputted into the driving transistors 321, each of from the first lightemitting element 301 a to the fourth light emitting element 301 d isdetermined to be a light emitting state or a non-light-emitting state.

At the same time of finishing writing to the source signal lines, thewrite period in the n-th line (n is natural number) is finished andturned to a store period. Then, an n-th plus 1 line turns to a writeperiod and write movement similar to the above is performed. Byrepeating the above-mentioned movement, write movement is performed fromthe first line to the last line.

A light emitting device to which the invention having theabove-mentioned structure is applied, relieves a burden on a lightemitting element having low luminous efficiency or a light emittingelement of a luminescent color to which the human eye has lowersensitivity. Accordingly, the deterioration of the light emittingelement or the reduction in image quality due to the deteriorated lightemitting element is suppressed. Further, more colors can be indicated;therefore, the range of color reproduction range of a light emittingdevice is expanded.

Embodiment 2

In this embodiment, a light emitting device in which arrangement oflight emitting elements in one pixel is different from the one shown inFIG. 1 is described

A light emitting device in this embodiment has a first light emittingelement 701 which emits red light, a second light emitting element 702which emits green light, a third light emitting element 703 which emitsblue light, and a fourth light emitting element 704 which emitsblue-tinged green light as shown in FIG. 7.

In the light emitting device in this embodiment, each of from the firstlight emitting element 701 to the fourth light emitting element 704 hasfour light emitting elements which are arranged in two rows and in twocolumns. A pixel 705 includes four light emitting elements, namely thefirst light emitting element 701 to the fourth light emitting element704. A plurality of the pixels 705 are arranged.

In the light emitting device in this embodiment, as the first lightemitting element 701, a first light emitting element 707, a first lightemitting element 708, and a first light emitting element 709 surroundedby a dotted line 706, four light emitting elements which indicate thesame color is included as one group. The four light emitting elementsare provided as a group two of which are each arranged in a row and acolumn. The group is provided for each of light emitting elements whichindicate the same color. In a fifth group where a first group includingfour light emitting elements which emit red light as the first lightemitting element 701, a second group including four light emittingelements which emit green light as the second light emitting element702, a third group including four light emitting elements which emitblue light as the third light emitting element 703, and a fourth groupincluding four light emitting elements which emit blue-tinged greenlight as the fourth light emitting element 704, are included as a group,the first group to the fourth group are arranged in two rows and in twocolumns. One group of one light emitting element included in the firstgroup, one light emitting element included in the second group, onelight emitting element included in the third group, and one lightemitting element included in the fourth group constitutes one pixel.

The width between light emitting elements included in a group isconstituted so as to be narrower than that of light emitting elementsincluded in different groups and adjacent to each other.

In other words, in the light emitting device of this embodiment, lightemitting elements are arranged so that the width between light emittingelements which have the same luminescent color and are adjacent to eachother is narrower than that of light emitting elements which havedifferent luminescent colors and are adjacent to each other.

When light emitting layers are formed by evaporation, the light emittinglayers which correspond to light emitting elements indicating eachluminescent color are individually formed by using a mask made frommetal or the like. At this time, the width of light emitting elementswhich have different luminescent colors and are adjacent to each otheris provided so as to be about from 20 μm to 30 μm, in order to preventlight emitting layers of light emitting elements which indicatedifferent luminescent colors from entering from edges and being mixedwith each other or the like.

However, by employing a light emitting device having a structure asshown in FIG. 7, there is no need for considering the prevention ofcolor mixture between light emitting elements which have the sameluminescent color and are adjacent to each other. Therefore, the widthbetween light emitting elements can be made narrower than that of lightemitting elements which have different luminescent colors and adjacentto each other. Accordingly, light-emission area (area of an electrodeside where emitted light can be extracted) of each light emittingelement can be expanded. As a result, luminous efficiency, particularly,light-extraction efficiency can be enhanced.

Although the pixels shown in FIG. 7 have square shapes, circular shapesor the like can be used without being limited thereto. When the pixelsare made to have circular shapes in particular, the effect that thelight emitting elements are not easily deteriorated can be obtained.

A light emitting device to which the invention having theabove-mentioned structure is applied, relieves a burden on a lightemitting element having low luminous efficiency or a light emittingelement of a luminescent color to which the human eye has lowersensitivity. Accordingly, the deterioration of the light emittingelement or the reduction in image quality due to the deteriorated lightemitting element is suppressed. Further, more colors can be indicated;therefore, the range of color reproduction range of a light emittingdevice is expanded.

Embodiment 3

In this embodiment, a cross-sectional structure of a light emittingdevice to which the present invention is applied is explained. However,the structure of the light emitting device according to the inventionand a substance constituting the light emitting device and the like arenot limited to those shown in this embodiment.

In FIGS. 8A to 8C, a transistor 11 surrounded by a dotted line isprovided for driving a light emitting element 12. The light emittingelement 12 corresponds to any one of the first light emitting element301 a to the fourth light emitting element 301 d shown in Embodiment 1,and the transistor 11 corresponds to any one of driving transistors 321a to 321 d shown in Embodiment 1. The light emitting element 12 includesa first electrode 13, a second electrode 14, and a light emitting layer15 sandwiched by these electrodes. A drain of the transistor 11 and thefirst electrode 13 are electrically connected to each other by a wiring17 which passes through a first interlayer insulating film 16 (16 a, 16b and 16 c). In addition, the light emitting element 12 is separatedfrom another light emitting element provided to be adjacent to eachother by a partition wall layer 18. In this embodiment, a light emittingdevice of the invention having such a structure is provided over asubstrate 10.

The light emitting layer 15 in the light emitting element 12 includes aplurality of layers in which a mixed layer including a luminous body anda substance having superior carrier transportability, a layer includinga substance having superior carrier (an electron, a hole)transportability, and a layer including a substance having superiorcarrier injectability are laminated. A luminescent color of the lightemitting element 12 is determined by a luminous body included in thelight emitting layer 15. Note that the combination of substances whichconstitutes the light emitting layer 15 may be different in each lightemitting element As the luminous body, the luminous body shown inEmbodiment Mode may be used. As the substance having superior electrontransportability in particular, among substances having superior carriertransportability, for example, a metallic complex having a quinolineframe or a benzoquinoline frame such as tris(8-quinolinolate)aluminum(abbreviation: Alq₃), tris(5-methyl-8-quinolinolate)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(abbreviation: BeBq₂),bis(2-methyl-8-quinolinolate)-4-phenylphenolate-aluminum (abbreviation:BAlq) or the like, can be given. As a substance having superior holetransportability, for example, a compound of aromatic amine system (inother words, a compound having the bond of benzene ring-nitrogen) suchas 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviation:α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylaniine(abbreviation: TDATA), or4,4′,4″-tris[N-(3-methylphenyl)N-phenyl-amino]-triphenylamine(abbreviation: MTDAXA) are given. In addition, as the substance havingsuperior electron injectability in particular, among substances havingsuperior carrier injectability, a compound of alkali metal oralkaline-earth metal such as lithium fluoride (LiF), cesium fluoride(CsF) or calcium fluoride (CaF₂) are given. In addition to these, amixture of a substance having high electron transportability such asAlq₃, and alkaline-earth metal such as magnesium (Mg) may be used. Asthe substance having high hole injectability, for example, metal oxidesuch as molybdenum oxide (MoOx), vanadium oxide (VOx), ruthenium oxide(RuOx), tungsten oxide (WOx) or manganese oxide (MnOx) are given. Inaddition to these, a phthalocyanine system compound such asphthalocyanine (abbreviation: H₂Pc) or copper phthalocyanine (CuPC) aregiven.

The transistor 11 is a top gate type. However, the structure of thetransistor 11 is not particularly limited, and for example, it may be aninverse stagger type as shown in FIG. 9A. When the transistor is aninverse stagger type, it may be a structure in which a protective filmis formed over a semiconductor layer forming a channel (a channelprotective type) as shown in FIG. 9B, or it may be a structure in whicha part of a semiconductor layer forming a channel becomes a concaveshape (a channel etch type). Reference numeral 21 denotes a gateelectrode; 22, a gate insulating film, 23, a semiconductor layer, 24, ann-type semiconductor layer, 25, an electrode; and 26, a protective film.

The semiconductor layer constituting the transistor 11 may be eithercrystalline or noncrystalline. Further, it may be semi-amorphous or thelike.

A semi-amorphous semiconductor is explained hereinafter. Asemi-amorphous semiconductor is a semiconductor having an intermediatestructure of an noncrystalline structure and a crystal structure(including monocrystal and polycrystal), and having a stable third statewith respect to free energy, and including a crystalline region having ashort distance order and lattice distortion. A crystal grain having agrain diameter of from 0.5 nm to 20 nm is included in at least oneregion of the semi-amorphous semiconductor film, and the Raman spectrumshifts to the lower side of wave number of 520 cm⁻¹. In addition, inx-ray diffraction, a diffraction peak of (111) and (220) derived from aSi crystal lattice is observed. The semi-amorphous semiconductor filmincludes hydrogen or halogen at least 1 atomic % or more as a neutralierof an uncombined hand (a dangling bond). Therefore, a semi-amorphoussemiconductor is also referred to as a microcrystal semiconductor. Thesemi-amorphous semiconductor film is manufactured by performing glowdischarging decomposition (plasma CVD) of a silicide gas. As thesilicide gas, SiH₄, additionally, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like can be used. The silicide gas may be diluted with H₂, or H₂and one or more of rare gas elements: He, Ar, Kr, and Ne. Dilution ratiois within the range of from 2 times to 1000 times. Pressure is roughlywithin the range of from 0.1 Pa to 133 Pa; power frequency, from 1 MHzto 120 MHz, preferably from 13 MHz to 60 MHz; and substrate heatingtemperature, at most 300° C. or less, preferably from 100° C. to 250° C.An atmospheric constitution impurity such as oxygen, nitrogen or carbonas an impurity element within a film is preferably 1×10²⁰ atoms/cm³ orless, in particular, oxygen concentration is 5×10¹⁹ atoms/cm³or less,preferably, 1×10¹⁹ atoms/cm³ or less. Note that a mobility of a TFI(thin film transistor) using a semiconductor having a semi-amorphoussemiconductor is about from 1 m²/Vsec to 10 cm²/Vsec.

As a specific example of a semiconductor layer having crystallinity, asemiconductor layer including monocrystal silicon or polycrystalsilicon, silicon germanium and the like are given. These semiconductorlayers may be formed by laser crystallization or for example,crystallization by a solid phase growth method using nickel or the like.

When the semiconductor layer is formed from noncrystalline substance,for example, from amorphous silicon, a light emitting device having acircuit in which the transistor 11 and the other transistor (atransistor constituting a circuit for driving a light emitting element)are all n-channel transistors, is preferable. As for other lightemitting devices, either a light emitting device having a circuitincluding either an n-channel transistor or a p-channel transistor, or alight emitting device having a circuit including both transistors, maybe used.

Further, the first interlayer insulating film 16 may be multi-layer asshown in FIG. 8A and 8C, or a single layer. A first interlayerinsulating film 16 a includes an inorganic material such as siliconoxide or silicon nitride, a first interlayer insulating film 16 bincludes a substance having self-planarization such as acrylic, siloxane(a substance which has a skeleton formed by the bond of silicon (Si) andoxygen (O), and which includes at least hydrogen as a substituent), orsilicon oxide which can be formed by application (spin coating). Notethat a substance which constitutes each layer is not particularlylimited and other substances not described here can be used. Inaddition, a layer formed from another substance may be further combined.Thus, the first interlayer insulating film 16 may be formed by usingboth an inorganic material and an organic material, or may be formed ofeither an inorganic film or an organic film.

It is preferable that a partition wall layer 18 has a shape in whichcurvature radius continuously changes in an edge portion. The partitionwall layer 18 is formed with acrylic, siloxane, a resist, silicon oxideor the like. The partition wall layer 18 may be formed from either aninorganic material film or an organic material film, or formed with theboth.

In FIGS. 8A and 8C, it is a structure in which only the first interlayerinsulating film 16 is provided between the transistor 11 and the lightemitting element 12. However, it may be a structure in which a secondinterlayer insulating film 19 (19 a and 19 b) are provided therebetweenin addition to the first interlayer insulating film 16 (16 a and 16 b),as shown in FIG. 8B. In the light emitting device shown in FIG. 8B, thefirst electrode 13 passes through the second interlayer insulating film19 and is connected to the wiring 17.

The second interlayer insulating film 19 may be multi-layer or a singlelayer as in the first interlayer insulating film 16. The interlayerinsulating film 19 a includes a substance having self-planarization suchas acrylic, siloxane (a substance which has a skeleton formed by thebond of silicon (Si) and oxygen (O), and which includes at leasthydrogen as a substituent), or silicon oxide which can be formed byapplication (spin coating). Further, the second interlayer insulatingfilm 19 b is formed from a silicon nitride film containing argon (Ar).Note that the substance which constitutes each layer is not particularlylimited and other substances not described here can be used. Inaddition, a layer formed from another substance may be further combined.Thus, the second interlayer insulating film 19 may be formed by usingboth an inorganic material and an organic material, or may be formed ofeither an inorganic film or an organic film.

In the light emitting element 12, in the case where both the firstelectrode 13 and the second electrode 14 are formed from a substancehaving light-transmitting properties such as indium tin oxide (ITO),emitted light can be extracted from both the first electrode 13 side andthe second electrode 14 side as indicated by outline arrows in FIG. 8AIn the case where only the second electrode 14 is formed from asubstance having light-transmitting properties, emitted light can beextracted from only the second electrode 14 side as indicated by anoutline arrow in FIG. 8B. In this case, it is preferable that the firstelectrode 13 includes a highly reflective material or that a filmincluding a highly reflective material (a reflective film) is providedunder the first electrode 13. In the case where only the first electrode13 is formed from a substance having light-transmitting properties,emitted light can be extracted from only the first electrode 13 side asindicated by an outline arrow in FIG. 8C. In this case, it is preferablethat the second electrode 14 is formed from a highly reflective materialor that a reflective film is provided over the second electrode 14.

In addition, the light emitting element 12 may have a structure in whichthe first electrode 13 functions as an anode while the second electrode14 functions as a cathode, or alternatively, a structure in which thefirst electrode 13 functions as a cathode while the second electrode 14functions as an anode. Note that the transistor 11 is a p-channeltransistor in the former case, and the transistor 11 is an n-channeltransistor in the latter case.

A light emitting device to which the invention having theabove-mentioned structure is applied, relieves a burden on a lightemitting element having low luminous efficiency or a light emittingelement of a luminescent color to which the human eye has lowersensitivity. Accordingly, the deterioration of the light emittingelement or the reduction in image quality due to the deteriorated lightemitting element is suppressed. Further, more colors can be indicated;therefore, the range of color reproduction range of a light emittingdevice is expanded.

Embodiment 4

In this embodiment mode, a light emitting element having a circuitstructure which is different from the one shown in FIG. 4 is describedwith reference to FIG. 10.

As shown in FIG. 10, circuits for driving each light emitting elementare connected to a light emitting element 801. The circuit includes adriving transistor 824 that determines a light-emitting ornon-light-emitting state of the light emitting element 801 in accordancewith an image signal, a switching transistor 822 that controls an inputof the image signal, an erasing transistor 823 that controls the lightemitting element 801 to be a non-light emitting state regardless of theimage signal, and a current control transistor 821 to control thecurrent value supplied to the light emitting element 801. Here, a source(or a drain) of the switching transistor 822 is connected to a sourcesignal line 831, a source of the driving transistor 824 and a source ofthe erasing transistor 823 are connected to a current supply line 832extending to juxtapose to the source signal line 831, a gate of theswitching transistor 822 is connected to a first scanning line 833, anda gate of the erasing transistor 823 extending to juxtapose to the firstscanning line 833 is connected to a second scanning line 834. Thecurrent control transistor 821 is sandwiched between the drivingtransistor 824 and the light emitting element 801 and serially connectedto one another. A gate of the current control transistor 821 isconnected to a power supply line 835. Note that the current controltransistor 821 is configured and controlled so that current flows in asaturation region in voltage-current (Vd-Id) characteristics.Accordingly, the intensity of the current value flown to the currentcontrol transistor 821 can be determined.

A driving method when the light emitting element 801 emits light isdescribed When the first scanning line 833 is selected in a writingperiod, the switching transistor 822 that has a gate connected to thefirst scanning line 833 is turned on. Then, an image signal inputted tothe source signal line 831 is inputted to a gate of the drivingtransistor 824 through the switching transistor 822. Further, currentflows from the current supply line 832 to the light emitting element 801through the driving transistor 824, and the current control transistor821 which becomes an on-state due to a signal from the power supply line835 to emit light At this time, the current value flowing to the lightemitting element is determined by the current control transistor 821.

The description in FIG. 10 is made about one light emitting element 801.However, in the light emitting device of this embodiment four lightemitting elements, each of which is a light emitting element operated bythe same circuit connected to the light emitting element are arranged asin FIG. 4. A plurality of pixels each of which has four light emittingelements as one group are arranged. Note that each light emittingelement included in a pixel indicates a different color.

FIG. 11 shows a top view of a pixel portion of a light emitting devicehaving a circuit structure as shown in this embodiment. In FIG. 11 onlya part of the pixel portion is illustrated. Note that a structure of apixel portion of a light emitting device is not limited to the structureshown in FIG. 11, and another structure may be used. In FIG. 11reference numeral 91 denotes a semiconductor layer, 92, a conductivefilm functioning as a gate (a gate electrode) of a current controllingtransistor 821, a switching transistor 822, an erasing transistor 823, adriving transistor 824, first scanning lines 833 and 834 and the like;and 93, a conductive film functioning as a source signal line 831, acurrent supply line 832, a power supply line 835 and the like. Inaddition, reference numeral 94 denotes a part which has a laminatedstructure in which a light emitting layer is sandwiched between a pairof electrodes.

A light emitting device to which the invention having theabove-mentioned structure is applied, relieves a burden on a lightemitting element having low luminous efficiency or a light emittingelement of a luminescent color to which the human eye has lowersensitivity. Accordingly, the deterioration of the light emittingelement or the reduction in image quality due to the deteriorated lightemitting element is suppressed Further, more colors can be indicated;therefore, the range of color reproduction range of a light emittingdevice is expanded.

Embodiment 5

The light emitting devices shown in Embodiments 1 to 4 to which thepresent invention is applied is mounted to a wide variety of electronicdevices after mounting an external input terminal and being sealed.

An electronic-device to which the invention is applied is an electronicdevice in which the deterioration of a light emitting element or thereduction in image quality due to the deteriorated element issuppressed, therefore, a preferable display can be obtained. Moreover,in the light emitting device, more colors can be indicated and colorreproduction range is expanded.

In this embodiment, a light emitting device to-which the invention isapplied and an electronic device to which the light emitting device ismounted are described with reference to FIGS. 12 and 13. Nonetheless,the light emitting device and the electronic device shown in FIGS. 12and 13 are just examples, and the structures of the light emittingdevice and the electronic device are not particularly limited thereto.

FIG. 12 is a cross-sectional view after sealing a light emitting deviceto which the invention is applied. A top frame format of FIG. 12corresponds to FIG. 3 described on ahead. A substrate 6500 and a sealingsubstrate 6501 are attached to each other with a sealant 6502 so as tosandwich a transistor and a light emitting element of the presentinvention therebetween. At the end of the substrate 6500, an FPC(flexible printed circuit) 6503 which is to be an external inputterminal is mounted. Note that the region sandwiched by the substrate6500 and the sealing substrate 6501 is filled with an inert gas such asnitrogen or a resin material.

One embodiment of an electronic device in which a light emitting deviceto which the invention is applied is mounted is shown in FIG. 13.

FIG. 13 illustrates a laptop personal computer manufactured by applyingthe invention, which includes a main body 5521, a chassis 5522, adisplay portion 5523 and a keyboard 5524 and the like. A display devicecan be completed by incorporating a light emitting device having a lightemitting element of the invention in a personal computer.

In this embodiment, although it is described about a laptop personalcomputer, in addition to this, a light emitting device having a lightemitting element of the invention may be mounted to a mobile phone, atelevision receiver, a car navigation system or a lighting device andthe like.

1. A light emitting device comprising a plurality of pixels arranged inmatrix, wherein the pixel comprising: a fist light emitting elementwhich emits red light; a second light emitting element which emits greenlight; a third light emitting element which emits blue light; and afourth light emitting element which emits a neutral color.
 2. A lightemitting device comprising a plurality of pixels arranged in matrix,wherein the pixel comprising a light emitting element which emits acolor having coordinates in the region where x in a chromaticity diagramis 0.15 or less, and y is 0.25 or more and 0.5 or less, when the coloris indicated by a CIE-XYZ color system.
 3. A light emitting devicecomprising a plurality of pixels arranged in matrix, wherein the pixelcomprising: a first light emitting element which emits a color havingcoordinates in the region where x in a chromaticity diagram is 0.6 ormore, and y is 0.35 or less, when the color is indicated by a CIE-XYZcolor system; a second light emitting element which emits a color havingcoordinates in the region where x in a chromaticity diagram is 0.3 orless, and y is 0.6 or more, when the color is indicated by a CIE-XYZcolor system; a third light emitting element which emits a color havingcoordinates in the region where x in a chromaticity diagram is 0.15 orless, and y is 0.2 or less, when the color is indicated by a CIE-XYZcolor system; and a fourth light emitting element which emits a colorhaving coordinates in the region where x in a chromaticity diagram is0.15 or less, and y is 0.25 or more and 0.5 or less, when the color isindicated by a CIE-XYZ color system.
 4. A light emitting devicecomprising a plurality of pixels arranged in matrix, wherein the pixelcomprising: a first light emitting element which emits red light; asecond light emitting element which emits green light; a third lightemitting element which emits blue light; and a fourth light emittingelement which emits a neutral color, wherein light emission brightnessof the first light emitting element, the second light emitting element,the third light emitting element and the fourth light emitting elementis independently controlled, and an image comprising multicolor isdisplayed by combining lights emitted from the first light emittingelement, the second light emitting element, the third light emittingelement and the fourth light emitting element.
 5. A light emittingdevice comprising a plurality of groups each of which includes fourlight emitting elements having the same luminescent color and arrangedin two rows and in two columns as one group.
 6. A light emitting devicecomprising a plurality of light emitting elements having differentluminescent colors, comprising: a first group including four first lightemitting elements which emit a first color, and the first light emittingelements are arranged in two rows and in two columns; a second groupincluding four second light emitting elements which emit a second color,and the second light emitting elements are arranged in two rows and intwo columns; a third group including four third light emitting elementswhich emit a third color, and the third light emitting elements arearranged in two rows and in two columns; and a fourth group includingfour fourth light emitting elements which emit a fourth color; and thefourth light emitting elements are arranged in two rows and in twocolumns, wherein the first group, the second group, the third group andthe fourth group are arranged in rows and columns two groups each, andthe width between light emitting elements included in the same group andadjacent to each other is narrower than that of light emitting elementsincluded in different groups and adjacent to each other.
 7. A lightemitting device comprising a first pixel and a second pixel which areadjacent to each other, wherein the first pixel includes a first lightemitting element which emits a first color, a second light emittingelement which emits a second color, a third light emitting element whichemits a third color and a fourth light emitting element which emits afourth color, the second pixel includes a fifth light emitting elementwhich emits the first color, a sixth light emitting element which emitsthe second color, a seventh light emitting element which emits the thirdcolor and a eighth light emitting element which emits the fourth color,the first light emitting element and the second light emitting elementare adjacent to each other, the first light emitting element and thefifth light emitting element are adjacent to each other, the secondlight emitting element and the sixth light emitting element are adjacentto each other, and the width between the first light emitting elementand the fifth light emitting element, and the width between the secondlight emitting element and the sixth light emitting element are narrowerthan the width between the first light emitting element and the secondlight emitting element.